Step wave power converter

ABSTRACT

A single- or multi-phase step wave power converter includes multiple transformers configured to receive DC voltage from one or more power sources. Each of the transformers includes a primary winding and a secondary winding. The transformers are each configured to supply a step for a step wave AC output. Bridge circuits are supplied for controlling input of DC voltage into the primary windings of the transformers. Steps for the step wave AC output are output from the secondary windings based upon the input provided to the primary windings. DC source management circuitry manages which DC power source(s) supplies DC voltage input to each of the bridge circuits. The management circuitry provides seamless power switching between the plurality of DC power sources based on each power source&#39;s performance characteristics. A pulse-width modulator can also be provided to the step wave power converter to modulate the input into a selected primary winding. In this way, the step wave AC output can be fine-tuned in substantial conformance with an ideal AC waveform.

[0001] This application is a continuation of Ser. No. 09/468,610, filedon Dec. 21, 1999

BACKGROUND OF THE INVENTION

[0002] This invention relates generally to step wave power convertersfor transforming power from power sources supplying DC voltage inputinto AC power. More specifically, this invention relates to step wavepower converters for providing greater input control over multiple DCpower buses and for more accurately simulating single- or multiple-phaseAC waveforms. While this invention is particularly directed to thetransformation of power from DC power sources to AC power, it should benoted that AC power sources can be readily converted to DC power sourcesthrough the use of a rectifier. Therefore, the scope of this inventionis not limited to strictly DC-to-AC power conversion.

[0003] Prior art patents and publications describe various single-phasestep wave power converters for transforming DC voltage input into a stepwave AC output. FIG. 1 is a schematic illustration showing one exampleof a prior art power converter. Referring to FIG. 1, one single-phasestep wave power converter of the prior art uses one transformer 2 foreach step of the step wave output. A single DC power source is used tosupply power to each of the transformers 2 in the power converter. Eachtransformer 2 has three windings, including two primary windings P1, P2and one secondary winding S. The two primary windings P1 and P2 areelectrically coupled to the DC power source through four gates G1-G4.The gates G1-G4 control the flow of current through the primary windingsP1, P2 in order to produce a step of the AC output from the secondarywinding S. The two primary windings P1, P2 in each transformer 2 areidentical to each other except that they are oppositely connected to theDC voltage source. Because of their reverse connections, they induceopposite polarity voltage in the secondary winding S. The secondarywindings S of the transformers are connected together in series so thattheir outputs can be combined to produce the step wave AC output.

[0004] In operation, the gates G1-G4 are controlled to alternately pulseDC current through the primary windings P1, P2. Current flow through apositive polarity primary winding P1 induces a positive step output fromthe corresponding secondary winding S, while, conversely, the flow ofcurrent through a negative polarity primary winding P2 induces anegative step. Steps from the secondary windings S of all of thetransformers 2 are added together to form the overall AC waveform.Consequently, pulsing DC current through the primary windings P1, P2 atthe appropriate time intervals causes the secondary windings S to outputan approximate AC waveform.

[0005] U.S. Pat. No. 5,373,433 issued to Thomas (Thomas) provides animprovement in the art with respect to single-phase power invertersoperating from a single DC power source. Specifically, Thomas disclosesswitching bridges for controlling DC voltage input into multipletransformers from a single source. Each switching bridge includes fourswitches arranged in two parallel lines, each of which has two seriesconnected switches. The switching bridges are controlled so that thetransformers produce either a positive, zero, or negative output voltagestep at a given time. According to Thomas, the transformers “preferablyhave turns ratios that are multiples of each other in order to provideboth good resolution and a wide dynamic range of the [AC output]signal.” Col. 5, 11. 58-62. For example, Thomas explicitly discloses asingle-phase power converter having three transformers capable ofproducing voltage outputs from their secondary windings of ±15V, ±45V,and ±135V, respectively. The output voltages from the secondary windingsof all of the transformers are combined in series. Thomas produces afairly accurate AC waveform by controlling timing and sequencing of thevoltage contributions from the three transformers to transitionsequentially through each of twenty-seven different possible overalloutput voltage levels. A special decoder circuit is also provided toprevent accidental shorting across the DC voltage input which wouldoccur if two switches in a series connected pair were closed at the sametime. Despite its improvements, Thomas does not contemplate either theuse of multiple power sources or three-phase operation.

[0006] Another prior art topology is described in U.S. Pat. No.5,631,820 issued to Donnelly et al. (Donnelly). Donnelly provides animprovement in the art by using three gates instead of four to controlcurrent flow through primary transformer windings. Also, although usingtransformers having two primary windings and one secondary winding,Donnelly's switching architecture allows each primary winding to be usedto produce either a positive or a negative step, rather than only one orthe other. Donnelly also provides an improvement in the art bycontemplating the use of multiple power sources, but fails to provideseamless integration and management of the multiple power sources basedon their performance characteristics. Donnelly also discloses athree-phase power converter topology that has nine gates and one threewinding, three-phase transformer per step.

[0007] Other prior art patents and publications also describethree-phase step wave power converters for converting DC voltage fromone or more DC power sources to a step wave AC output. Referring to FIG.2, one example of a prior art three-phase step wave power converterincludes multiple three-phase transformers 4, each having three windings(two primary P1, P2 and one secondary S) per phase per step. Theconfiguration of each phase is similar to the single-phase arrangementof the prior art described above with reference to FIG. 1. Each phase ofeach transformer includes two primary windings P1, P2 and a secondarywinding S. The two primary windings P1, P2 of each phase are identicalto each other except for their opposite connections to the DC powersource. Four switches G1-G4 are used to control current flow through theprimary windings P1, P2 of each phase. The switches are used toalternately pulse DC voltage through the primary windings P1, P2 inorder to generate steps of the AC waveform for that particular phasefrom a corresponding secondary winding S. The contributions output fromthe secondary windings S of the transformers for a given phase arecombined together in series to produce the step wave AC output for thatphase.

[0008] Unfortunately, this prior art configuration is bulky, requiring athree winding, three-phase transformer 4 controlled by 12 gates for eachstep. Also, each primary winding P1, P2 contributes only one positive orone negative step towards the overall AC waveform output and the totalnumber of steps of the AC output directly corresponds to the number ofprimary windings used to produce the output. To get better resolution inthis three-phase AC waveform output, therefore, more transformers mustbe added to the system, further increasing its bulkiness.

[0009] It should be noted that in each of the prior art three-phase stepwave converters, the three-phase transformers 4 used are wye-wyetransformers, meaning that both the primary P1, P2 and secondarywindings S are arranged in wye configurations. This configuration ispresumably used to avoid voltage contention which occurs between deltaand wye connections in delta-wye transformers.

[0010] A further drawback of each of the prior art power converters isthat the step wave AC output is generally blocky as a result of the mereaddition of positive and/or negative block steps to form the AC waveformoutput. Although blocky AC waveforms are acceptable for manyapplications, they are less than desirable for use in many modemelectronic devices such as computers, televisions, etc., which performbetter and last longer when power is supplied to them using a closelyregulated AC power supply.

[0011] Therefore, the industry faces several problems related toconventional step wave power conversion. First of all, the industry hasbeen unable to seamlessly integrate power from multiple power sourcesbased on their performance characteristics. The industry has also failedto produce a step wave AC output that more closely approximates an idealAC waveform. Additionally, the industry has been unable to produce athree-phase step wave AC power output in a more efficient manner. Theindustry has further failed to enhance the resolution of the AC waveformoutput from a three-phase step wave power converter without increasingthe number of primary transformer windings. Furthermore, the industryhas not succeeded in allowing a single power source to selectivelysupply power to multiple transformers when other power sources becomedisabled or go offline. Nor has the industry succeeded in preventingbackfeed to the power grid or in allowing any DC power source connectedto the converter to be charged from any of the other power sourcesconnected thereto.

[0012] Accordingly, the industry would be benefitted by a step wavepower conversion method and apparatus which provides seamlessintegration between multiple power sources. The industry would befurther benefitted by a step wave AC output which more closelyapproximates an ideal AC waveform. The industry is in further need of amore efficient step wave power converter. The industry would also bebenefitted by a method of converting DC voltage into three-phase poweroutput with enhanced resolution with simpler circuitry. The industry isin still further need of a step wave power converter which allows asingle power source to selectively supply power to multiple transformerswhen other power sources become disabled. Still further needs in theindustry include preventing backfeed to the power grid and allowing anyDC power source with storage capability connected to the converter to becharged from any of the other power sources connected to the converter.

SUMMARY OF THE INVENTION

[0013] According to the needs of the industry, one object of the presentinvention is to seamlessly integrate power from multiple power sourcesbased on their performance characteristics.

[0014] Another object of the present invention is to produce a step waveAC output that more closely approximates an ideal AC waveform.

[0015] Another object of the present invention is to produce three-phaseAC power output in a more efficient manner.

[0016] Yet a further object of the present invention is to enhance theresolution of the step wave power output from a three-phase step wavepower converter without increasing the number of transformer components.

[0017] Still another object of the present invention is to selectivelyallow a single power source to supply power to multiple transformerswhen one or more other power sources become disabled.

[0018] Further objects of the present invention include preventingbackfeed from the DC power buses to the input power grid and allowingany of the DC power sources connected to the converter to be chargedfrom any of the other power sources connected to the converter.

[0019] This invention provides a significant improvement in the art byenabling an improved step wave power converter for converting DC voltageinput into a step wave AC output. The step wave power converter of thisinvention is provided with multiple transformers configured to receiveDC voltage from a plurality of power sources. Each of the transformersincludes a primary winding and a secondary winding. The transformers areeach configured to supply a step for a step wave AC output. Bridgecircuits are supplied for controlling input of DC voltage into theprimary windings of the transformers. Steps for the step wave AC outputare output from the transformer secondary windings based upon the inputprovided to the primary windings. Source management circuitry manageswhich power source(s) supplies DC voltage to each of the bridgecircuits. The management circuitry provides seamless power switchingbetween the plurality of power sources based on each power source'sperformance characteristics. The step wave AC output can be a single- ormulti-phase AC output. A pulse-width modulator can also be provided tothe step wave power converter to modulate the input into a selectedprimary winding. In this way, the step wave AC output can be fine-tunedin substantial conformance with an ideal AC waveform.

[0020] A three-phase step wave power converter according to oneembodiment of this invention includes multiple three-phase transformers.Each three-phase transformer has primary and secondary windings. Thethree-phase transformers are configured to receive DC voltage from oneor more power sources into their primary windings and to supply one ormore steps for each phase of a three-phase step wave AC output fromtheir secondary windings. A plurality of bridge configurations orcircuits are also supplied, each of which is made up of multiple gatepairs arranged in parallel. Each gate pair includes two or more gatesarranged in series. Opposite ends of each of the primary windings ofeach transformer are connected between gates in separate gate pairs of acorresponding bridge. Each bridge circuit is thereby configured tocontrol current flow across the primary windings of its correspondingtransformer. Preferably, the transformers are arranged having adelta-wye configuration in which primary windings are coupled in a deltaarrangement and secondary windings are arranged in a wye configuration.When configured in this way, the resolution of the three-phase step waveAC output can be enhanced by managing characteristics of the voltagetransformation between the delta primary winding configurations and thewye secondary winding configurations.

[0021] A method for enhancing a three-phase step wave AC output from athree-phase step wave power converter is also provided. The three-phasestep wave power converter has multiple three-phase transformers havingprimary and secondary windings, with each transformer arranged in adelta-wye configuration. The method begins by receiving one or more DCvoltage inputs into the step wave power converter. Steps of thethree-phase step wave AC output are generated from the secondarywindings by controlling timing and sequencing of the DC voltage inputsinto the primary windings. Voltage phase characteristics of thedelta-wye transformation are managed to increase the number of steps inthe three-phase step wave AC output.

[0022] Yet another embodiment of the invention provides a step wavepower converter similar to those previously described, but which alsoincludes cross-tie circuitry to allow one of the power sources to supplypower to two or more transformers when one or more of the other powersources becomes unstable, inoperative, or goes offline. This cross-tiecircuitry includes gated connections between two or more of the DCbuses. Each power source can further be provided with cut-off gates toallow it to be readily disconnected from the input system(s).

[0023] A still further embodiment of a step wave power converterincludes an isolation switch for isolating at least one of the powersources from the input power grid to prevent or gate backfeed to thegrid. It should also be noted that isolation switches can be providedfor each of the power sources, to isolate each of them from each of theother power sources as well as from the input power grid. When each ofthe power sources are isolated from each other, bi-directional circuitrycan further be provided for allowing any of the DC power sources to becharged from any other of the power sources. Providing isolated powersources also allows DC power to be supplied by a rectified variablefrequency and voltage input.

[0024] Finally, a method for enhancing the characteristics of a stepwave AC output from a step wave power converter is provided in which aDC voltage is supplied to the step wave power converter. The DC voltageis transformed into a plurality of steps of the step wave AC output.Significantly, the DC input voltage is pulse-width modulated such thatthe step wave AC output more closely approximates an AC waveform. Thismethod works particularly well when DC input voltages are provided tomultiple transformers and when the input voltage to a selected one ormore of the transformers is pulse-width modulated while holding theinputs to one or more of the other transformers in a constant on or offstate. This allows fine-tuning of the step wave AC output in substantialconformity with an ideal AC waveform.

[0025] It will be readily apparent to those of skill in the art that theabove described features and advantages can be combined in numerous waysnot limited to those combinations explicitly described herein.Furthermore, the foregoing and other objects, features, and advantagesof the invention will become more readily apparent from the followingdetailed description of preferred embodiments of the invention whichproceed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a schematic illustration of a conventional single-phasestep wave power converter for converting DC voltage from a single powersource into a step wave AC output.

[0027]FIG. 2 is a schematic illustration of a conventional three-phasestep wave power converter for converting DC voltage from a single powersource into a three-phase step wave AC output.

[0028]FIG. 3 is a schematic diagram of a single-phase step wave powerconverter configured to receive and manage DC voltage inputs frommultiple power sources according to one embodiment of the presentinvention.

[0029]FIG. 4 is a series of graphs illustrating the generation of asingle-phase step wave AC output from a step wave power converter, suchas the one illustrated in FIG. 3, having four transformers.

[0030]FIG. 5A is a schematic illustration of a step wave powerconverter, similar to the one illustrated in FIG. 3, further includingcross-tie circuitry and cut-off gates for selectively providing ordisabling input from one of the DC power sources to one or moretransformers according to another embodiment of the present invention.

[0031]FIG. 5B is a schematic illustration of a step wave powerconverter, similar to the one illustrated in FIG. 3, for controllingpower inputs from multiple sources according to yet another embodimentof the invention.

[0032]FIG. 6 is a schematic illustration of a three-phase step wavepower converter according to yet another embodiment of the presentinvention, showing an improved bridge arrangement and delta-wyetransformer configurations.

[0033]FIG. 7A is a more detailed schematic illustration of thethree-phase step wave power converter according to FIG. 6, furthershowing insulated gate bipolar transistor (IGBT) modules containingbridge circuitry, driver boards for driving the bridge circuitry,control boards for controlling the driver boards, and series connectionsbetween secondary windings of the transformers for each phase, amongother things.

[0034]FIG. 7B is an enlarged view of the transformer configuration ofthe converter in FIG. 7A.

[0035]FIG. 7C is a block diagram of a control board having both softwareand hardware components for controlling the step wave power converter ofFIG. 7A according to a preferred embodiment of the present invention.

[0036]FIG. 8 is a voltage versus time graph showing a step wave ACoutput from a delta-wye three-phase step wave power converter similar toFIG. 8A but having enhanced resolution resulting from careful control ofvoltage characteristics in the delta-wye transformers.

[0037]FIG. 9 is a flow chart illustrating a method for more accuratelyapproximating an ideal AC waveform using a hybrid of step wave powerconversion and pulse-width modulation according another embodiment ofthe invention.

[0038]FIG. 10 is a voltage ratio versus time graph illustratingoperation of the hybrid step wave and pulse-width modulation powerconversion method of FIG. 9.

[0039]FIG. 11A is a schematic illustration of a prior artuninterruptible power supply system for providing backup power.

[0040]FIG. 11B is a schematic illustration of the step wave powerconverter of the present invention for use as a backup power systemaccording to yet another embodiment of the invention.

DETAILED DESCRIPTION

[0041] The step wave power converter (SWPC) of this invention is aninnovative power converter designed around a unique platform that allowsit to have a wide range of uses beyond those of conventional powerconverters. These uses extend beyond the usual task of converting powerfrom a single DC source to AC power. One such use includesconsolidation, integration and supervisory control of multiple powersources through a single SWPC while isolating each source so that eachcan operate at optimum efficiency. The power sources connected to theSWPC can include diesel or gas generators, wind turbines, solarphotovoltaic (PV) cell arrays, hydroelectric generators, batteries, gasturbine generators, fuel cells, etc. Yet another use is in backup powersupply systems, including integration, isolation, and management of thepower sources that comprise the backup power supply system. Stillanother use is managing the power for power generators installed in thedistributed generation mode. Another use is end of grid and in linevoltage and power quality regulation. Further uses include standard 60Hz or customized frequency regulation; the ability to feed reactivepower to a grid or an off-grid load on demand; and the provision of aprogrammable microprocessor controller that is customized and optimized,as required, for each application.

[0042] Specific embodiments of the present invention will now bedescribed in more detail. FIG. 3 is a schematic illustration of asingle-phase step wave power converter for receiving and managing DCvoltage inputs from multiple power sources according to one embodimentof the present invention. According to this embodiment, DC buses 5receive power from the power sources and supply it as a DC voltage inputto one or more bridge circuits 10. Each bridge circuit 10 preferablyconsists of an insulated gate bipolar transistor (IGBT) module havingfour IGBT switching gates G1-G4, which are controlled by a driver boardin response to signals from a control board. Each IGBT switching gateG1-G4 is preferably fitted with an antiparallel diode D1-D4,respectively, to allow shorting current to flow. Although IGBT switchinggates are preferred, the gates can include HEXfets or othersemiconductor power switching devices and a corresponding antiparalleldiode. In this embodiment, a single two winding (one primary P and onesecondary S) transformer 15 is used for each step.

[0043] Single-phase shorting using the four gate bridge 10 involvesclosing the two gates G1, G2 on the positive inputs (the positivetransistors) or the two gates G3, G4 on the negative inputs (thenegative transistors). Closing the gates in this manner allows shortingcurrent to flow through one diode and one gate of a shorted transformer15, thereby imposing a null potential across the primary winding P ofthe shorted transformer. Shorting is important for allowing powersupplies to be dynamically added or removed from a transformer withoutaffecting the transformer's winding ratio requirements.

[0044]FIG. 4 illustrates the production of a single-phase step wave ACoutput from a step wave power converter, such as the one described abovewith reference to FIG. 3. Referring now to FIG. 4, a step wave AC outputis produced as follows. In a step wave power converter having fourtransformers, each transformer produces an output from its secondarywinding according to a voltage input into its primary winding and thetransformer winding ratio. Each of these outputs forms a building block,or step, of an overall AC output. The outputs from all of the secondarytransformer windings are added together in series to simulate the ACsine wave.

[0045] Generally, the process for producing the step wave outputproceeds by turning on each of the transformers sequentially at aspecified time and then leaving them on for a given period of timebefore sequentially deactivating them. Specifically, this process beginsby turning on a first transformer at a zero reference time t0. Theactivation of the first transformer activates step one of the step waveoutput. Step one remains activated while other steps are added. At afirst point in time t1, a second transformer is turned on and itsvoltage output is combined with the output of the first transformer,thereby activating step two. Similarly, at a second point in time t2, athird transformer is turned on and its voltage output is added to thatof the others to activate step three of the step wave output. Likewisealso, at a third point in time t3, a fourth transformer is turned on toactivate step four.

[0046] At a later specified time, the step wave production process isreversed to step the AC waveform back down. This is accomplished bysequentially turning the transformers off at fourth, fifth, sixth, andseventh points in time t4, t5, t6, and t7. Turning a transformer offpreferably includes shorting a voltage across the primary winding of thetransformer as described above. Although this reverse process canproceed by turning off the transformers in any order, a preferred methodproceeds by deactivating the transformers in the order they wereactivated. Accordingly, the first transformer is deactivated first, thesecond transformer next, and so on. Specifically, step one isdeactivated by turning off the first transformer at the fourth point intime t4. Step two is deactivated at the fifth point in time t5 byturning off the second transformer. Similarly, step three is deactivatedat the sixth point in time t6 by turning off the third transformer. And,finally, step four is deactivated at the seventh point in time t7 byturning off the fourth transformer. By deactivating the transformers inthe order they were activated, balancing of the transformer duty cyclesis achieved.

[0047] Although not shown, after all of the transformers have beenturned off, the waveform building process is repeated in order to buildthe later 180 degrees (or negative half) of the AC sine wave. Theprocess for producing the negative half of the waveform is the same asfor the positive half just described, except with negative voltagepolarity.

[0048] Referring again to FIG. 3, a positive step for the AC waveform isgenerated by closing the first positive switch G1 and the secondnegative switch G4 in one of the bridge circuits. A negative step forthe AC waveform is generated by closing the first negative switch G3 andthe second positive switch G2 in one of the bridge circuits. A shunt ofthe transformer primary P is generated by closing either both positiveswitches G1, G2 or both negative switches G3, G4.

[0049] In summary, the steps of the simulated AC waveform of FIG. 4 areproduced by sequentially enabling and disabling DC voltage input intothe primary windings of multiple transformers at specified points oftime. In some embodiments, each step may be formed from the voltagecontributions of only one transformer. In other embodiments, however,each step may be formed from the voltage contributions of more than onetransformer.

[0050] Voltage control of the step wave AC output is established byvarying the number of transformers active at any given time as well asthe duty cycle associated with each of those transformers. Thetransformers can also be sized to assure that any number less than thetotal number of transformers are capable of producing rated outputvoltage. Additionally, by adding steps and by varying the duty cycle ofany given step, a wide range of output voltages can be derived.Additionally, step width can be varied to generate the proper waveformand RMS voltage.

[0051] When multiple power sources are provided to a step wave powerconverter, it is sometimes desirable to maintain the ability to crossconnect any of the power sources to any number of transformers in theconverter. Consequently, according to a preferred embodiment of thisinvention, each power source connected to the SWPC is supplied with abypass switch. Bypass switches allow the SWPC to switch off an abnormalpower source. Bypass switches further allow the SWPC to prevent backfeedto the grid. Bypass switches can be added to any SWPC configuration bysupplying cut-off gates to the power source input lines as illustratedin FIG. 5A. This allows the gating mechanism of the grid source to beblocked when needed. Another layer of protection can be achieved usingthe cross-tie approach described below.

[0052]FIG. 5A shows additional DC source management circuitry includinga cross-tie arrangement for interconnecting multiple power sources withmultiple transformers. In the cross-tie arrangement, a step wave powerconverter is provided with gated interconnections, called cross-ties,between DC buses 5. The gates on the interconnections are referred to ascross-tie gates. The cut-off gates described above are included on eachpower source's positive and negative input lines to isolate that powersource from the other power sources and the grid. In normal operation,the power source cut-off gates are closed to allow power to be suppliedfrom each of the power sources while the cross-tie gates are open toprovide isolation between DC buses. When one of the power sources failsor is disconnected, however, a degrading DC bus 5 is sensed. The cut-offgates associated with the failed source then open to isolate and preventfurther contribution of power from the compromised power source, and thecross-tie gates close to allow a still functioning power source tosupply power to the DC bus 5 for the compromised power source. Thiscontrol mode assures seamless transfer between power sources while stillmaintaining isolation between them. Although FIG. 5A shows only twopower sources, it should be appreciated that this embodiment is scalableto include any number of power sources and cross-tie devices. Therefore,more than two sources can be added in this scheme.

[0053] An improvement in the art realized by yet another embodiment ofthis invention results from the provision of bi-directional circuitrybetween the isolated power sources. Bi-directional circuitry betweenisolated power sources gives the SWPC of this invention the ability tocharge any of the DC sources connected to the SWPC from any of the othersources connected to the SWPC. In other words, this circuitry enables abi-directional capability on any of the DC buses 5 but maintains theirisolation from one another. For example, in a SPWC where a battery and aphotovoltaic (PV) cell array comprise two of the power sources, thebattery can be charged from the PV array while still maintaining thearray's isolation from the battery. This is a significant innovationbecause the batteries can stay at a relatively constant voltage whilethe PV maximum power point voltage fluctuates.

[0054] A still further advantage provided by the use of isolated DCbuses is the ability of the SWPC of this invention to allow variablespeed operation of any combination of rotating or fixed power generationmeans. For example, a variable speed diesel, a variable speed windmill,and a PV array can all be run through a single SWPC when isolation ismaintained between each of the DC buses. In other words, when thediesel's rectified DC bus is isolated from the rectified DC bus of thewindmill and the PV array, each of the sources can operate at anydesired speed or voltage level without interfering with the othersources.

[0055]FIG. 5B illustrates still other embodiments of this inventionwhich are also configured to provide multiple power source managementthrough power source management circuitry. One such embodiment includesmultiple high frequency input converters tied to each DC bus 5, whileyet another includes a combined multiple input converter. By using morethan one high frequency input converter tied to each DC bus 5, or acombined multiple input converter, each input may contribute as much ofthe power to the overall system as desired. The top circuit illustratedin FIG. 5B shows multiple high frequency input converters tied to eachDC bus 5. In this embodiment, power inputs from each of the multiplepower sources are run through a separate isolation circuit which canalso contain pulse width modulation circuitry. One of the input powersources, i.e., Input #1, can be an input power grid. The outputs fromall of the isolation circuits are combined together and supplied to theDC bus 5. Each DC bus 5 can then be used to supply power to atransformer. The transformer would receive DC voltage input from the DCbus, which receives power from one or more power sources, including theinput power grid, through the isolation circuit. The isolation circuitcan thereby isolate the DC bus from the input power grid to preventbackfeed to the grid from the DC bus.

[0056] The circuit illustrated at the bottom of FIG. 5B illustrates acombined multiple input converter tied to a DC bus 5. In this converter,multiple high frequency DC/DC, PFC, and AC/DC converter inputs frommultiple sources may be converted to a common DC bus 5. By providingproper feedback control, each input can supply a regulated portion ofthe power used in the inverter. The portion of the power supplied byeach input can be adjusted by the control board. This feature can alsobe incorporated into a single, high frequency converter circuit, withmultiple inputs, that synchronizes control and reduces components.

[0057] Multiple power source management is particularly beneficial wheresome or all of the power sources produce non-uniform power outputs, suchas photovoltaic cells, windmills, etc. According to this invention, suchsources could be used to provide a large amount of the power when theirstrength is high, but be used to supply less of the power as theyweaken. The control signal for each input converter will determine theamount of power transferred from each power source. This embodimentthereby facilitates “soft” transfers between input sources. Unlike“hard” power transfers, where a power source is either connected to ordisconnected from the system, “soft” power transfers allow each powersource to contribute a desired percentage of the power to each of thetransformers. Also, this invention allows power sources to be slowlyramped-in or ramped-out when being connected to or disconnected from thesystem, helping to prevent voltage spikes and provide a more uniformpower supply. These types of multiple power source control can beutilized with either single-phase or three-phase power converters.

[0058] Still another embodiment of this invention provides improvementsin the art specifically with respect to three-phase step wave powerconverters. This embodiment is the unique SWPC configuration shown inFIG. 6. FIG. 6 is a schematic illustration of a three-phase step wavepower converter including an improved bridge architecture 20 anddelta-wye transformers 25. Specifically, this embodiment utilizes aunique step wave power converter topology consisting of multiplethree-phase transformers 25, each arranged with primary windings PA, PB,PC in a delta configuration and secondary windings SA, SB, SC in a wyeconfiguration. Voltage flow across the primary windings PA, PB, PC ofeach transformer is controlled by six gates G1-G6 configured in a bridgecircuit 20. One or more power sources can be used to supply power to thebridge circuits 20 through their respective DC buses 5. Each of thegates G1-G6 in a bridge circuit 20 includes an insulated gate bipolartransistor (IGBT) fitted with an antiparallel diode to allow shortingcurrent to flow. A primary benefit of this new topology is that itrequires only six gates G1-G6 and one three-phase transformer 25 perstep (having only one primary and one secondary winding per phase)rather than the nine or twelve gates and the more complex transformerconfigurations of the prior art.

[0059] As mentioned above, the connections between the primary windingsof each of the three-phase transformers 25 in this embodiment arearranged in a delta configuration. Each three-phase transformer 25includes a single primary winding PA, PB, or PC and a single secondarywinding SA, SB, or SC for each phase. In the delta configuration, afirst end of a phase A primary winding PA and a second end of a phase Cprimary winding PC of one transformer 25 are coupled together andconnected to that transformer's bridge circuit 20 between two gates G1and G4 in a first series connected gate pair. Similarly, a second end ofthe phase A primary winding PA and a first end of the phase B primarywinding PB are coupled together and connected to the bridge circuit 20between two gates G2 and G5 in a second series connected gate pair.Finally, a first end of the C phase primary winding PC and a second endof a B phase primary winding PB are coupled together and connected tothe bridge circuit 20 between two gates G3 and G6 in a third seriesconnected gate pair. The secondary windings SA, SB, SC of eachthree-phase transformer 25 are arranged in a wye configuration, with allof the secondary windings SA, SB, or SC of the same phase beingconnected together in series.

[0060] Operation of the three-phase transformers 25 using the six gatebridge 20 will now be described in more detail. Voltage across theprimary windings PA, PB, PC of the transformers is controlled to inducethe steps of the AC waveform output for each phase A, B, C through thecorresponding secondary windings SA, SB, SC. Each of the transformers 25directly contributes one step to the AC output of each phase.Specifically, when a voltage is applied across a transformer's primarywinding corresponding to one of the phases, the corresponding secondarywinding produces a step for that phase of the AC output. Furthermore,similar to the single-phase embodiment, a voltage is shorted across oneor more of the primary windings of the three-phase transformer 25 inorder to shunt them. Three-phase shorting (i.e., shorting of all threephases) using a six gate bridge 20 involves closing the three positivetransistors G1-G3 or the three negative transistors G4-G6. Closingeither set of three gates allows shorting current to flow through acombination of diodes and gates so that a null potential is imposedacross all three primary windings PA, PB, PC of the shorted transformer.

[0061] Furthermore, each of the primary windings PA, PB, PC of thetransformer 25 may have a potential or be shunted at different timesbased upon the operation of the six gates G1-G6. For instance, the phaseA primary winding PA will be on when either of two sets of gates G1, G5or G2, G4 are closed. Positive polarity voltage is applied across thephase A primary winding PA when a first positive gate G1 and a secondnegative gate G5 are closed. Conversely, reverse polarity voltage isapplied to the phase A primary winding PA when a second positive gate G2and a first negative gate G4 are closed. Phase A is shorted and turnedoff, however, when either the two positive gates G1, G2 or the twonegative gates G4, G5 connected to opposite ends of the phase A primarywinding PA are closed. Similarly, the phase B primary winding PB will beon when either of two sets of gates G2, G6 or G3, G5 are on. Positivepolarity voltage is applied across the phase B primary winding PB whenthe second positive gate G2 and a third negative gate G6 are closed.Reverse polarity voltage is applied across the phase B primary windingPB when a third positive gate G3 and the second negative gate G5 areclosed. The phase B winding PB is turned off when either the twopositive gates G2, G3 or the two negative gates G5, G6 connected to itsopposite ends are closed. Phase C is again similar. Positive polarityvoltage is applied across the phase C primary winding PC when the thirdpositive gate G3 and the first negative gate G4 are both closed. Reversepolarity voltage is applied across the phase C primary winding PC whenthe first positive gate G1 and the third negative gate G6 are closed.Finally, the phase C primary winding PC is turned off when either thetwo positive gates G1, G3 or the two negative gates G4, G6 connected toits ends are closed.

[0062] It should be appreciated that the gates G1-G6 may be controlledin any number of combinations in order to produce the desired steps foreach phase. Accordingly, by controlling the six gates G1-G6 of thebridge circuit 20, voltage of either positive or negative polarity or anull potential can be applied across the primary windings for eachphase. In this way, the desired contribution to the overall AC waveformcan be output from the phase's corresponding secondary winding based oncontrol of the bridge circuit 20.

[0063]FIG. 7A is a detailed schematic illustration of a three-phase stepwave power converter (SWPC), such as the one described above withreference to FIG. 6. FIG. 7B is an enlarged view of the transformerarrangement of the SWPC of FIG. 7A. Referring to FIG. 7A, IGBT modulesprovide the bridging circuitry 20 for control of DC power into theprimary transformer windings of three-phase transformers. Power issupplied from a power source to DC buses 5. The DC buses 5 supply DCvoltage input to terminals N and P of each IGBT module 20, whereterminal N is the DC negative terminal and terminal P is the DC positiveterminal. Each of the IGBT modules 20 produces three separate outputs A,B, and C from its three output terminals U, V, and W. These outputs arethe building blocks for the A, B, and C phases of the three-phase ACoutput.

[0064] In this embodiment, four IGBT modules 20 are used to control whenDC voltage inputs are supplied to the primary transformer windings offour three-phase transformers 25 to produce the steps (or buildingblocks) for each of the three phases. Of course, more or fewer than fourIGBT modules 20 and transformers 25 could be used. The ratio betweenIGBT modules 20 and transformers 25 is typically one to one. Eachthree-phase transformer 25 includes three primary windings and threesecondary windings (one of each for each phase). Also, in thisembodiment, each IGBT module 20 is supplied power from a single,separate DC power bus 5, each of which is connected to its own powersource (Power Sources 1-4). It should be appreciated, however, that anynumber of power sources may be connected to any one or more of the DCbuses 5, as has been described above with respect to other embodimentsof the invention.

[0065] The IGBT modules 20 regulate the flow of current from their DCbus 5 across the primary windings of their corresponding transformers 25in order to produce the steps of the three-phase AC waveform. The fourIGBT modules 20 are each controlled by one of four driver boards 22 thatare, in turn, controlled by a control board 24. More specifically, acontrol algorithm, resident on the control board 24, controls signalssent to each of the four driver boards 22 that, in turn, send signals toactivate the gates inside of each of the four IGBT modules 20 at theappropriate times. The control algorithm thereby controls the activationof the IGBTs in a desired sequence to produce the step wave AC output.

[0066] Referring now to FIG. 7B, outputs A, B, and C from the IGBTmodules 20 are fed to the primary windings PA, PB, PC of theircorresponding three-phase transformers T1-T4 to control voltage therein.Each transformer T1-T4 directly produces a single step for each phasefrom its secondary windings SA, SB, SC based on current flowing throughits corresponding primary windings PA, PB, PC. The four transformersT1-T4 are configured having secondary windings of the same phaseconnected together in series. The three phases are also connected into awye configuration on each of the secondaries.

[0067] As mentioned above, the three-phase step wave power converter ofthis embodiment has control circuitry including three types of controldevices, as shown in FIG. 7A. The control board 24 has all programmedinformation and is the heart of the control system. The driver boards 22are an interface between the control board 24 and the IGBT modules 20.The IGBT modules 20 are the power electronics that allow the electricalside of the step wave power converter to operate. The IGBT modules 20are preferably commercially available Powerex six pack modules made byPowerex Intellimod and are unmodified from their original condition. Thedriver boards 22 are generally known to those skilled in the art. Thecontrol board 24 is being designed and built specifically for use withthe single- and three-phase step wave power converters of thisinvention, a schematic representation of which is shown in FIG. 7C.Resident on the control board 24 is the micro-controller chip that isused to control all aspects of the step wave power converter. Thesoftware in the control board 24 enables the unique switching aspects ofthe invention.

[0068] The control board software manages the operation of the entireSWPC. It controls the operation of all of the IGBT switches within eachof the IGBT modules 20 that in turn characterize the AC waveform. Theproper timing for operating each of the switches in each IGBT module 20is crucial to generating acceptable AC power quality. The software alsoprovides features such as the ability to maintain loading for individualinput sources; control of AC output voltage and current; phasing andgrid synchronization; the ability to monitor and isolate gates and orgate driver logic failures; the ability to skew step wave timing toreduce harmonic distortion; step wave/pulse-width modulation (PWM)hybrid control; the ability to combine multiple inputs with differentvoltages and ratings; the ability to provide feedback to power sourcesto allow following of output loading; and the ability to allow heavyloading of single inputs, such as batteries, for a short period of timeduring transient conditions to allow for sources with a slower reactiontime to pick up loads. Although the foregoing and other features arepreferably implemented by software, it should be noted that some or allof these aspects of the invention may be performed in analog circuitryrather than by software.

[0069] In a basic three-phase step wave power converter, each phase ofthe AC waveform can be constructed in the same way as was describedpreviously with respect to the single-phase step wave AC output shown inFIG. 4. Although the basic stepping procedure works very well with thesingle-phase power converter, however, using it directly on thethree-phase power converter of this invention produces contentionbecause of the delta-wye configurations of the gates and transformers.This contention is very detrimental to the AC power output quality.Remarkably, however, in the three-phase delta-wye configuration, thetiming of the IGBT switches can actually be controlled in such a way(i.e., to adjust phase shift instead of step width) that the transformerphase shift is used in a constructive, rather than a destructive,manner. A phase management controller, such as the control board 24, maybe used to control the switches to use transformer phase shiftconstructively.

[0070] Therefore, although the arrangements of the primary and secondarywindings of three-phase transformers can be configured many differentways, it becomes advantageous in this invention to configure the primarywindings in a delta arrangement and the secondary windings in a wyearrangement. Specifically, this delta-wye arrangement, when properlycontrolled, allows the resulting step wave to be made to contain n+2steps, where n is the number of transformers involved in producing theAC waveform. The AC waveform thus produced includes additional stepscaused by the addition of three phase-shifted primary waveforms.

[0071]FIG. 8 is a voltage versus time graph showing an enhanced stepwave AC output produced by carefully controlling the IGBT switches touse phase shift between delta and wye transformer winding configurationsconstructively. As shown, constructive use of transformer contention canprovide a six step waveform using only four transformers, rather thanthe conventional four step waveform.

[0072] To obtain the improved results described above, the inventionprovides a unique method for controlling the IGBT switches that allowsthe three-phase step wave power converter to produce an AC outputvoltage with higher resolution. This higher resolution includes anincreased number of output voltage steps while using the same number ofIGBT switches and transformers. This unique control method combines thenormal phase shift combinations in the delta-wye transformerconfiguration with smart transformer phase shift control logic to reduceharmonics. In other words, by intelligently activating and deactivatingthe switches on the IGBT boards according to the natural delta-wye phaseresponse, the step wave power converter of this embodiment provides anenhanced step wave AC output signal.

[0073] As discussed previously, use of conventional step wave switchingalgorithms to produce a simulated AC output is well-known. It is alsoquite common for conventional inverters to utilize a pulse widthmodulation (PWM) switching algorithm to approximate a sine wave. PWMrefers to the change of the on and off times (duty cycle) of pulses,such that the average voltage is the peak voltage times the duty factor.In such PWM inverters, a sine wave is approximated using a series ofvariable-width pulses. None of the prior art power converters, however,have combined a step wave output with PWM. A significant improvement inthe art is provided by this invention through a novel combination ofstep wave power conversion and PWM.

[0074] Fortunately, both step wave and PWM processes can cycle powersources in any sequence as well as control individual input sourceloading. Several advantages therefore result from the combination ofthese two approaches. These advantages include, among other things,closer approximation to a sine wave than with either of the prior artapproaches alone, fewer losses than in conventional pulse-widthmodulation approaches, elimination of the need for rapid switching offull line voltage, and greater adaptability of the AC waveform output.

[0075] Accordingly, still another preferred embodiment of this inventionutilizes a unique combination of step wave and PWM algorithms togenerate a hybrid step wave/PWM AC output that very closely approximatesan ideal AC sine wave (i.e., potentially less than 2% total harmonicdistortion). FIG. 9 is a flow diagram of a preferred algorithm forcombining PWM with step wave power conversion. This flow chartillustrates a process for creating a hybrid step wave/PWM waveform thatclosely approximates an ideal AC waveform. It should be noted that thisalgorithm can be incorporated as firmware on a micro-controller withsupporting analog circuitry or it can be completely analog or completelymicro-controller based.

[0076] Generally, according to this novel approach, PWM is used toimprove the transition edges of each step of a step wave AC output. Thehybrid step wave/PWM system uses a pulse-width modulator to modulate thepower input into a selected one of the transformers while inputs intothe other transformers are held in a steady on or off position, tomaintain the basic steps of the AC step waveform. PWM waves are therebyused in the step wave transitions to refine the envelope of thesimulated AC waveform. These smaller PWM pulses can be filtered to helpproduce a well regulated sine wave that has very little harmonicdistortion. In this way, the step wave process is used to approximate anAC sine wave on a large scale while the PWM process provides higherrefinement to the sine wave approximation. The combination of using PWMfor one or more transformers while using step wave power conversiontechnology for others is unique.

[0077]FIG. 10 is a graph that further illustrates the hybrid process forcreating an AC waveform as described above using the algorithm of FIG.9. The vertical axis of the graph represents the ratio of the totalvoltage output V(out) from series combined secondary transformerwindings of the SWPC to a peak voltage Vsetpoint of an ideal sine wave.The horizontal axis is a time axis. The bottom graph represents the PWMoutput supplied to a selected primary winding of one of thetransformers. Generally, as illustrated by the graph of FIG. 10, thehybrid step wave/PWM approach works by adding the step waves together togenerate a rough estimate of a sine wave while pulse-width modulatingvoltage input signals during transitions to smooth the edges of thesteps.

[0078] Referring to FIGS. 9 and 10, the hybrid step wave/PWM algorithmwill now be described in detail. First, however, the parameters of thealgorithm need to be defined. V(i) is used to represent the voltageapplied at a primary winding i, where i=1, 2, 3, 4, . . . , k; and wherek represents the total number of transformer primary windings used ingenerating the AC waveform. As noted previously, V(out) represents thecombined output voltage from the series connected secondary transformerwindings and Vsetpoint indicates the maximum voltage level of an idealAC waveform. The PWM envelope represents the limits within which the PWMoperation takes place, such that the unfiltered pulses are bound by thePWM envelope.

[0079] When the hybrid step wave/PWM process begins, the combined outputvoltage V(out) is at zero and the parameter i is set to 1. PWM of a DCinput voltage V(1) into a first primary winding of a first transformertherefore begins. Accordingly, the input voltage V(1) begins to begradually supplied to the first primary winding, such that the firsttransformer is turned on. As the voltage V(1) supplied to the firstprimary winding is modulated and filtered, it gradually increases, asshown by the PWM output graph 26 at the bottom of FIG. 10. The outputvoltage from the first transformer's secondary winding and the combinedoutput voltage V(out) increase correspondingly. This input voltage V(1)is continuously modulated as shown by signal 28 until the PWM levelreaches 100% for that step at time 30. Once PWM for that step reaches100%, the input voltage V(1) into the first primary winding iscontinuously turned on, as represented by line 32, and the parameter iis then incremented by one so that an input voltage V(2) into a secondprimary winding of a second transformer can then be modulated, asrepresented by signal 34.

[0080] The PWM process described above is repeated for the voltageinputs to each of the primary windings until the last required primarywinding k is reached. When this occurs (i.e., when i becomes equal tok), pulse width modulation of the input voltage V(k) into the finalprimary winding begins and continues until the overall output voltageV(out) becomes equal to the maximum voltage Vsetpoint of the ideal ACwaveform. When the output voltage V(out) reaches this point, it is atits maximum desired value and must therefore begin to be decreased. Todecrease the output voltage V(out), i is reset to 1 and the PWM processis reversed.

[0081] It should be noted that during modulation of each of the voltagesteps, the combined output voltage level V(out) is continuously testedto see if it has reached its maximum desired value Vsetpoint. As long asthe output voltage V(out) remains below the maximum, however, PWM of thecurrent step continues until it reaches 100% for that step, as describedabove. When the voltage output level V(out) reaches its maximum desiredvalue, i is reset to 1 and the PWM process is reversed so that thevoltage can be gradually reduced, whether or not all of the primarywindings have been used.

[0082] The PWM process continues by gradually reducing PWM of inputvoltage V(1) to zero, as represented by signal 36. Once PWM reaches 0%for that step, its input voltage V(1) is turned off continuously and iis incremented by one so that the input voltage V(2) into the secondprimary winding can be modulated, as represented by signal 38. Thisprocess continues for the voltage inputs for each of the primarywindings 1-k as they are each gradually reduced to zero. After PWM ofthe final input voltage V(k) (signal 40) has been completed and thevoltage output V(out) is zero (at time 44), the parameter i is againreset to 1. The entire process then repeats, except this time withnegative polarity as shown in 42.

[0083] As a result of the hybrid step wave/PWM process described above,it is believed possible to create a simulated AC waveform with less than2% total harmonic distortion. This invention therefore provides asignificant improvement in the art by enabling a SWPC which produces asimulated AC waveform which very closely approximates an ideal ACwaveform.

[0084] A few specific applications for this invention will now bedescribed further. One specific application for the use of an SWPChaving multiple, controllable, isolated source inputting is in hybridrenewable power systems. SWPCs of this invention can seamlessly andefficiently integrate renewable energy sources such as hydro, wind, andsolar power with conventional generators such as diesel and gas turbinesin off-grid, end-of-grid, and on-grid applications without compromisingthe efficient operation of the conventional or the renewable powergenerator units. Using such an SWPC permits the renewable power sourcesto be used as the primary sources while still ensuring continuousoperation, thereby reducing fuel consumption of the conventional powergenerators.

[0085] Yet another use for the present invention is in backup powersystems. Backup power systems are used to provide power to facilitieswhen the utility grid fails. These systems usually consist of a dieselgenerator (the primary power supply when operating off-line), batteriesthat provide temporary power during generator start-up, a power inverterthat inverts the DC battery or generator output to AC power, and astatic switch that transfers the load from the utility grid to thebackup power supply when needed. This entire system is conventionallyreferred to as an uninterruptible power supply (UPS). Unfortunately,most UPS systems suffer from one significant shortcoming—if one of themajor components fails, the entire system is compromised.

[0086] More specifically, in a typical UPS system, such as the one shownin FIG. 11A, a utility grid 50 and a backup power system (generator) 52are not synchronized. A transfer switch 56 selects between the two powersource input lines #1 and #2 depending on which power source 50 or 52 isdesired. When the utility grid 50 fails, the backup power line #2 isactivated to supply power from the backup power source 52. One or morebatteries 54 provide temporary DC power that is inverted to AC power forthe user. After the generator 52 comes to normal operating speed, powerwill be provided solely by the generator 52. A rectifier 57 is used torectify the power from the utility grid 50 or the generator 52 to DCpower. An inverter 58 inverts the incoming DC power to AC power. Becauseof this interdependent component configuration, if any one of thecomponents fails, the entire system is compromised.

[0087] Unlike the conventional system, the SWPC of this invention, whenused in a UPS application as schematically illustrated in FIG. 11B, canaccommodate and integrate multiple power sources 50, 52, 54. The abilityto integrate multiple power sources gives the SWPC 18 importantadvantages over the typical UPS systems. First of all, the inventioneliminates the need for the transfer switch 56 (see FIG. 11A) that isused with many UPS systems. This invention therefore provides trulyseamless “uninterruptible” power. This invention also preferablyisolates each power source 50, 52, 54 from the system to providecontinuous voltage regulation. If one of the power sources, such as theutility grid 50, becomes inactive or is deliberately disconnected, thisembodiment of the invention will regulate the power output using theremaining power sources 52, 54. This feature can eliminate costly downtimes by allowing scheduled service of power supplies without affectingthe user. When combined with the rugged and reliable design of the SWPC18 relative to commercial inverters that reside within the UPS, thisarchitecture is much more reliable and useful than a typical UPS system.

[0088] The SWPC 18 can also condition the power from the utility gridthat is to be used with sensitive electronics—a process thatconventionally requires additional equipment provided by the end user.This provides improved efficiency, regulation, and isolation over theuse of ferro-resonant transformers, as are conventionally used. Theflexibility of the SWPC 18 also gives the end user room for expansion ormodernization of power sources. For example, an existing dieselgenerator 52 or battery bank 54 could be replaced with fuel cells asthey become available.

[0089] Yet a further application of this invention is in integratingpower from photovoltaic (PV) cell or battery arrays. PV cells andbatteries are power sources for which the SWPC of this invention isideally suited. This is because these DC sources are typically made upof multiple, independent “strings”. PV arrays, for example, typicallyconsist of multiple strings of PV cells. Larger battery banks are alsotypically arranged as parallel strings of batteries and will benefitfrom use of the SWPC. Each string delivers power as a DC output voltage.The SWPC 18 can treat each string as an independent source andelectrically integrate the multiple strings, while maintaining isolationbetween them. This is a key advantage of the SWPC 18 because if one ormore strings malfunction, the SWPC 18 can continue to deliver utility-or electronic-grade AC power from the still-functioning strings.

[0090] Additionally, the SWPC 18 of this invention can cater to variousnominal DC voltage levels among the strings. Existing inverter systemsdeal with nominal DC voltage levels through individual voltageregulators on each string or by merging all of the DC power on a singleDC bus and then inverting the power from the bus to AC power. In somecases, inverters are attached to each string and the AC power from eachinverter is combined to feed the load. The SWPC 18 drasticallysimplifies and improves the power conversion architecture compared withprior art inverters for PV arrays allowing maximum power point trackingof each input.

[0091] A still further application of this invention exists with respectto fuel cells. Fuel cells create electricity using an electrochemicalprocess. They differ from batteries, which also use an electrochemicalprocess, in that they consume hydrogen and must therefore have fuelcontinuously provided. The type of fuel used to generate hydrogen variesand depends on the reforming process for which each system is designed.Fuel cells are well suited for distributed generation, but each systemmust be tailored to the application that it will serve. Someapplications may require higher power quality than others; some may needto be interconnected with the utility grid; some may require severalfuel cells to be paralleled together; while some may implementco-generation where waste heat of the fuel cell is used along with theelectrical energy. All these applications require power conditioning andcustom electrical interconnections with the end user's facility.

[0092] The electricity generated from fuel cells is also DC and must beregulated or converted to AC for user consumption. Conventionally, thisis accomplished by using a power converter that is often not integratedinto the design. The SWPC 18 of this invention offers clear advantagesover present techniques. One primary advantage in fuel cell integrationoffered by the SWPC 18 is parallel operation of multiple fuel cellswhere each unit may be individually loaded. Another advantage is theability to follow fuel cell voltage versus loading curves and limits.

[0093] Two conventional methods exist for consolidating the power ofmultiple fuel cells servicing a single user. One method is to usevoltage regulators for each fuel cell and to have a common bus to whichthese voltage regulators feed power. The power from the common bus isthen converted by a single power converter and fed to the user. A secondmethod is to use a power converter for each fuel cell, combine theconverted power and feed it to the user. Both of these techniques arecostly because of the duplication of system components for each fuelcell.

[0094] Referring again to FIGS. 11A and 11B, the SWPC 18 has a distinctadvantage over the prior art described above. Specifically, one or morepower sources, i.e., 52, 54, for the SWPC 18 can be a fuel cell. Byreplacing the battery banks 54 with fuel cells, the SWPC 18 allows eachfuel cell 54 to operate at peak efficiency by isolating each fuel cell54 from the others, as with the other power sources already describedabove. The SWPC 18 converts the power to AC and supplies it to the user.This simplifies the architecture and allows one or more of the fuelcells 54 to be taken off-line without any adverse effects.

[0095] Still further benefits of this invention exist with respect togrid-connected applications that have one or more inputs from the grid.In present input grid-connected applications, the fuel cell 54 isconnected to an inverter in synchronization with the utility grid 50. Itis disconnected from the grid 50 (i.e., for servicing) using a transferswitch 56. The SWPC 18 of this invention offers a clear advantage overthe transfer switch 56. Both the utility grid 50 and the fuel cell 54,or multiple fuel cells 54, are used as power sources for the SWPC 18.The SWPC 18 conditions the power for the user and isolates each fuelcell 54 from the utility grid. The SWPC 18 allows each fuel cell 54 tooperate under the preferred conditions for fuel efficiency orco-generation. In addition, since all power sources are isolated by theSWPC 18, there is no need for a costly transfer switch 56 in the eventone of the sources fails. The SWPC 18 will simply use the remainingpower sources 50, 52, 54 to create high quality power.

[0096] Fuel cells 54 can also provide power when the grid 50 is notactive or when there is no grid available. When power from the utilitygrid 50 is lost, the fuel cell 54 will provide backup emergency powerfor the user. For UPS systems, the fuel cell 54 can effectively replacethe diesel generator source 52, which is commonplace today.

[0097] The invention's ability to integrate multiple power sourcesfurther gives the end user the ability to expand the system powercapacity in the future without costly system upgrades or the purchase ofan entirely new system. With simple software modifications, the SWPC 18of this invention can be upgraded to accommodate multiple fuel cells,interconnection with the utility grid, or to parallel other types ofpower sources with the fuel cell.

[0098] Having described and illustrated the principles of the inventionin several preferred embodiments thereof, it should be apparent that theinvention can be modified in arrangement and detail without departingfrom such principles. Particularly, the features and advantages of allof the various embodiments can be arranged together in any combination,depending only on the desired application. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

We claim:
 1. A step wave power converter comprising: a plurality of transformers configured to receive DC voltage from a plurality of power sources, each transformer comprising a primary winding and a secondary winding and being configured to supply a step for a step wave AC output; a plurality of bridge circuits for controlling DC voltage inputs into the primary windings in order to output steps for the step wave AC output from the secondary windings; and source management circuitry for managing how the DC voltage inputs are switched by the bridge circuits, according to each power source's performance characteristics. 